Growth rates in a molecular beam epitaxy system can be inferred from measured beam fluxes. In principle, this can be done during deposition by placing an ion gauge of some sort in the periphery of the beam. More typically, beam fluxes from effusion cells are measured prior to deposition and assumed to remain constant.
Actual growth rates of layers can be measured by noting the oscillations in Reflection High Energy Electron Diffraction (RHEED) pattern intensities as a function of time after growth is initiated. RHEED is ideally suited for analyzing growing layers since it does not involve apparatus which would tend to block the molecular beams. In particular, the study of RHEED intensity oscillations caused by the periodic variation in the density of monolayer terrace edges has uncovered many aspects of the layer by layer epitaxial growth process and has allowed precise determination of structures grown. (See: Cohen et al., J. Vacuum Sci. Technol., A4, p.1251, 1986; Dobson et al., J. Crys. Growth, 81, p.1, 1987).
In the case of beam flux measurements, it is necessary to convert the flux into actual molecular layer growth rates. For many substances, evaporation from the growing wafer surface of one or more component is important and makes the correlation between growth rate and beam flux quite complicated and unusable.
RHEED intensity oscillations successfully circumvent this problem by actually measuring growth rates of molecular layers. However, to get stable RHEED patterns from which to measure intensity oscillations, the wafer must be mounted in a stationary position relative to the incident electron beam. This requires that the wafer not be rotating. However, due to nonuniformity in growth rates across the substrate position for all current source geometries highly non-uniform growth rate layer growth results without rotation. In fact, substrate rotation was introduced into MBE systems for just this reason. Thus, there is an inherent incompatibility between the needs of the RHEED measurement and the needs for layer uniformity.
An additional problem is the damage to the substrate which has been observed caused by the RHEED electron beam. Furthermore, RHEED is insensitive to some of the properties of the crystal surface which are important to an understanding of the nature of the crystal being grown.